1. Field of the Invention
This invention relates to a method of controlling a printer using a dc motor as a drive source.
2. Prior Art
A conventional printer of this type employs a control method in which a time is set so that a stopped dc motor is started to reach a predetermined speed irrespective of the length of time the motor power was turned off after the motor power is turned on again to perform the print/sheet-forward (printing and sheet forwarding) operations for a succeeding line after the motor power was turned off upon the end of the print/sheet-forward operations for the last line. The print/sheet-forward operations for the succeeding line are started after such set time elapses.
FIG. 5 is a timing chart showing an operation method of an ordinary conventional printer. After a motor has been started by a motor drive signal 50, a timing signal 51 or the like is detected. Based on the detected timing signal, a printing operation 53 is performed at a predetermined position and sheet forwarding operation 54 is then performed. The print/sheet-forward operatings are repeated for a required number of lines. The motor drive signal 50 is then turned off to stop the dc motor.
A method of controlling the conventional printer will be described with reference to the flowchart shown in FIG. 8 and the timing chart shown in FIG. 9. In Step 11 the stopped dc motor is started by applying the motor drive signal 50, and in Step 12 a timer for regulating a start time A within which the motor reaches a predetermined speed E is started. After the start time A, as marked by a timer output signal 55, has elapsed in Step 13, the print/sheet-forward operations are performed in Step 14. When the print/sheet-forward operations have been completed in Step 14, it is judged in Step 15 whether the print/sheet-forward operations are to be stopped or to be continuously performed. If the print/sheet-forward operations are to be continuously performed, the processing returns to Step 14. If the operation is to be stopped, the motor drive signal 50 is turned off to stop the dc motor in Step 16.
FIG. 7 shows a waveform of the motor current between the start and stop of the printer as shown in FIGS. 8 and 9. The vertical axis indicates a motor current I and the horizontal axis indicates a time T. A large current G flows when the motor is started since the motor has been stopped. As the motor is accelerated, the current G is gradually decreased. The current value increases during the print operation portion H and the sheet-forward operation portion J due to increases in mechanical loads, and the motor is shut off thereafter. If large loads at a print operation portion H and a sheet-forward operation portion J shown in FIG. 7 are applied to the dc motor before the dc motor reaches the predetermined speed, the motor cannot overcome the loads. As a result, the dc motor is stopped, and the printer is also stopped during the operation. To prevent such accidental stoppage of the printer, the print/sheet-forward operations are performed in Step 14 of FIG. 8 after the start time A, marked by the timer output signal 55 shown in FIG. 9, has elapsed.
FIG. 6 shows waveforms indicating motor speeds between the start and stop of the printer shown in FIGS. 8 and 9. The vertical axis indicates a motor speed V and the horizontal axis indicates a time T. Since the dc motor is shut off at a start portion D, the motor speed starts from zero. The motor speed is gradually increased, and reaches a predetermined value E after a start time A has elapsed. When the motor speed has reached the predetermined value E, the print/sheet-forward operations are performed, and thereafter the motor power is turned off. As a result, the motor is decelerated and stopped as shown by the solid line in portion F. The time required for the dc motor to come to a complete stop is usually several times the start time A, within which the dc motor reaches the predetermined speed E from zero, due to inertia in the rotor section of the dc motor. When the motor power is turned on again to perform the print/sheet-forward operations for a succeeding line during this stop operation F, the motor speed reaches the predetermined value E within a short period of time, as shown by the broken line in portion M, unless the motor speed V is zeroed.
However, the above-mentioned conventional printer has addressed the following shortcomings.
(1) When the motor power is turned on again to perform the print/sheet-forward operations for a succeeding line, even if the dc motor continues to rotate due to inertia in the rotor immediately after the motor power has been turned off upon completion of the last print/sheet-forward operations, it is necessary to set a timer for a predetermined time within which the dc motor reaches a predetermined speed from the stopped state, thereby waiting for such predetermined time to expire before starting the print/sheet-forward operations for the succeeding line. Thus, as long as the time required for the print/sheet-forward operations are concerned, the speed for the intermittently-driven print/sheet-forward operations for single lines is slower than speed for the continuous print/sheet-forward operations.
(2) If the dc motor speed is increased to reduce the delay in the print/sheet-forward speed during the print/sheet-forward operations, the print/sheet-forward mechanism is subjected to large loads. As a result, increased cost due to increased power consumption and use of heavy-duty parts capable of accommodating high speed operations are encountered.
(3) To prepare print data and the like for a succeeding line within the print/sheet-forward time to allow continuous printing, an LSI chip including a high-speed processing CPU must be provided, which entails large costs also in designing control circuits.